This disclosure relates to improvements in forming chromized coatings through overlay and diffusion coating processes.
Substrates that are subject to corrosion, such as gas turbine engine components, may include a coating to protect an underlying material from corrosion. Hot section components commonly found in the combustor and turbine sections of modern gas turbine engines are made of high temperature alloys selected from the group consisting of nickel-based superalloys, iron based superalloys, cobalt-based superalloys and combinations thereof. These superalloys have been developed to meet the demands of higher operating temperature while being able to survive the severe environment of the hot section of the gas turbine. In order to improve the survivability of the component at high temperatures under corrosive and oxidative conditions, protective coating systems typically are applied to the components.
Current corrosion resistant coatings generally fall into two categories, overlay and diffusion. Both exhibit compositions that are rich in chromium in order to support the formation of protective chromium oxide scales. Due to access, only diffusion type coating is available in the internal passageways of the turbine airfoils. Further, the presence of the diffusion coating on portions of the airfoil that get subsequent overlay oxidation resistant coating has been shown to be detrimental to both coatings. It can be difficult to mask areas effectively where coating is not required. Chromizing before or immediately after the bond coat deposition has been shown to compromise the performance of the coatings. The need exists for a way to use both diffusion chromizing for internal and external overlay coatings without compromising the performance of either of the coatings.